This invention relates to semiconducting titanate ceramic thermistors and more particularly to such thermistors having fatigue resistant joints between the thermistor body and the leads.
Semiconducting ceramic thermistors typically have a positive temperature coefficient of resistance (PTCR) and a distinct temperature threshold at which the resistance of the thermistor changes several orders of magnitude. Such thermistors are usually employed in a self heating switching mode to control circuit currents as in a motor start control circuit. Thus the PTCR thermistor typically experiences a rapid change in temperature during each cycle of circuit operation wherein a temperature change of around 100.degree. C. commonly occurs in a fraction of a second. Such thermistors often include two electrodes being bonded to separate regions of the ceramic body with a lead wire or other terminal means being attached to each electrode.
Electroding metals that have been found suitable for making low resistance ohmic contact with the semiconducting thermistor body have generally been limited to such metals as nickel and aluminum. A base film or layer of these contacting metals is usually coated with a highly solderable silver containing over layer. Lead wires are then attached to the electrodes by soldering. Troublesome and difficult to deposit high temperature solders are often necessary since thermistors are typically required to operate at temperatures from 140.degree. to 200.degree. C.
Alternatively, lead wires are attached to the base layer of ohmically contacting electrode metal by means of a silver loaded resin that is subsequently heat cured. It is also known to employ an electroding paste containing a large quantity of silver particles, a small quantity of aluminum particles and a glass frit. The silver reduces the resistivity of the electrodes which may be advantageous when using thin film electrodes. However, such silver bearing electrodes thicker than about 0.001 inch (0.0025 cm) are not practical at all for most purposes because they are too costly.
Another alternative is to provide spring loaded metal plate terminations against each contacting electrode film as is described in the U.S. Pat. No. 2,835,434 issued Sept. 10, 1974 and 3,914,727 issued Oct. 21, 1975, both of which are assigned to the same assignee as in the instant invention.
Conventional electroding and associated lead soldering methods consist of a plurality of manufacturing steps that are rather complex. But of greater significance is the fact that thermistors having soldered leads are capable of being cycled no more than about 10,000 times before the solder joint becomes fatigued, presents a higher and higher ohmic resistance and ultimately fails physically. Thermistors with resin-silver bonded leads exhibit a similar lead joint fatigue failure. Thermistors having spring loaded plate terminations do not exhibit such a terminal fatigue failure mode and have longer lives by orders of magnitude. However, the latter represents more complex package designs and tends to be more costly.
It is therefore an object of the present invention to provide a simple leaded PTCR thermistor having a long useful life.
It is a further object of the present invention to provide a simple method for making a leaded PTCR thermistor.
It is a further object of this invention to provide a low cost PTCR thermistor having substantially silver free electrodes.
It is a further object of this invention to provide a simple PTCR thermistor capable of more than 10,000 self-heating switching cycles without failure.